Shining ring around black holes recreated in the lab

An image of the black hole at the centre of our galaxy, showing the glowing accretion disc. Credit: Event Horizon Telescope

Imperial researchers have created a spinning disc of plasma in a lab, mimicking discs found around black holes and forming stars.

The experiment more accurately models what happens in these plasma discs, which could help researchers discover how black holes grow and how collapsing matter forms stars.

As matter approaches black holes it heats up, becoming plasma – a fourth state of matter consisting of charged ions and free electrons. It also begins to rotate, in a structure called an accretion disc. The rotation causes a centrifugal force pushing the plasma outwards, which is balanced by the gravity of the black hole pulling it in.

Understanding how accretion discs behave will not only help us reveal how black holes grow, but also how gas clouds collapse to form stars. Dr Vicente Valenzuela-Villaseca

These glowing rings of orbiting plasma pose a problem – how does a black hole grow if the material is stuck in orbit rather than falling into the hole? The leading theory is that instabilities in magnetic fields in the plasma cause friction, causing it to lose energy and fall into the black hole.

The primary way of testing this has been using liquid metals that can be spun, and seeing what happens when magnetic fields are applied. However, as the metals must be contained within pipes, they are not a true representation of free-flowing plasma.

Now, researchers at Imperial have used their Mega Ampere Generator for Plasma Implosion Experiments machine (MAGPIE) to spin plasma in a more accurate representation of accretion discs. Details of the experiment are published today in the journal Physical Review Letters.

Accelerating plasma

First author Dr Vicente Valenzuela-Villaseca completed the study during his PhD in the Department of Physics at Imperial, funded by a President’s Scholarship. He said: “Understanding how accretion discs behave will not only help us reveal how black holes grow, but also how gas clouds collapse to form stars, and even how we might be able to better create our own stars by understanding the stability of plasmas in fusion experiments.”

The team used the MAGPIE machine to accelerate eight plasma jets and collide them, forming a spinning column. They discovered that the closer to the inside of the spinning ring was moving faster, which is an important characteristic of accretion discs in the Universe.

MAGPIE produces short pulses of plasma, meaning only around one rotation of the disc was possible. However, this proof-of-concept experiment shows how the number of rotations could be increased with longer pulses, allowing better characterisation of the disc’s properties. A longer experiment run time would also allow magnetic fields to be applied, to test their influence on the friction of the system.

Dr Valenzuela-Villaseca said: “We are just as the start of being able to look at these accretion discs in whole new ways, which include our experiments and snapshots of black holes with the Event Horizon Telescope. These will allow us to test our theories and see if they match astronomical observations.”

Finding funding

Dr Valenzuela-Villaseca is from Chile, and he found very few opportunities for funding he could apply to when looking to conduct his PhD in the UK. He was awarded an Imperial President’s PhD Scholarship, which he said was crucial in allowing him to work with MAGPIE.

He is now based at Princeton University in the US, continuing to work on laboratory astrophysics experiments, but maintains links with the MAGPIE team to make sure the experimental and theoretical programmes feed into each other.

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‘Characterization of Quasi-Keplerian, Differentially Rotating, Free-Boundary Laboratory Plasmas’ by V. Valenzuela-Villaseca et al. is published in Physical Review Letters.